The present invention is related to an apparatus for electrodeposition of a metallic coating onto a metallic strip and, more particularly, to a device for improving the transfer of current to the strip in a radial cell type electroplating apparatus.
Steel strip is used in many applications which are subject to conditions which could lead to corrosion, such as body panels on motor vehicles and exterior building panels. In order to improve the corrosion resistance of steel strip, it is often plated with a corrosion resistant material, such as zinc or a zinc alloy. While this coating may be applied through a hot dip process, superior coating adhesion, paintability and formability are obtained through electroplating of the metallic material onto the strip.
Apparatus for electroplating can be of several primary types: horizontal, vertical, or radial. The present invention is directed to a device for use on a radial cell electrodeposition apparatus. In this apparatus a large rotating drum is used as the cathode and the strip is directed into as tank containing electrolyte and is passed about the circumference of the cathodic drum. Electrical current is caused to flow from one or more anodes through the electrolyte solution to the strip as the strip passes through the electrolyte bath about the exterior of the rotating drum cathode. In order to prevent plating of metal on the drum side of the strip, deflector rolls above the electrolyte bath urge the strip into contact with the radial drum in sealing engagement therewith.
The amount of current which is delivered to the cell determines the thickness of the coating plated onto the strip during its immersion within the electrolyte bath. In order to apply a thicker coating or to increase the speed at which the strip travels through the cell while applying a predetermined thickness of coating, higher electrical current is required. In order to achieve the high plating speed required for commercial electrodeposition operations, a relatively high current density must be applied to the strip. If this current is not evenly transferred to the strip, areas of very good contact between the strip and the conductor band of the conductor drum can experience local heating which can result in very small areas of strip discoloration, which are called "hot spots", or of strip deformation, which are called "arc spots". Since this material is usually intended for exterior applications, the customer specifications are very rigid and result int he rejection of material which exhibits even very light defects of this nature. In order to avoid these defects, the plating line may be run at a slower speed than optimum, resulting in productivity losses.
One method of improving the uniformity of the contact between the strip and the conductor band is to increase the tension on the strip pull it more tightly around the conductor drum, which urges it more firmly against the conductor band. However, all steel strip is of a relatively light gauge and therefore has a relatively low yield stress. The tension required to achieve acceptable strip-conductor band contact is just below the yield stress of standard strip gauges (about 0.005 to 0.010 inch [0.13 to 0.25 mm] thick) and is above the yield stress of relatively thin gauge strip and of strip of low yield stress steel grades, such as interstitial free (IF) steels which are used for drawing. Therefore, these steels can not be coated effectively using this procedure.
What is needed is an improved apparatus for use in a radial-type electroplating cell to improve the transfer of current between the conductor band and the strip to prevent current-induced defects and improve productivity while reducing the tension required for effective current transfer.